1. Field of the Invention
The present general inventive concept relates to a method of manufacturing an ink-jet printhead, and more particularly, to a method of manufacturing a monolithic inkjet printhead by photolithography using a photoresist.
2. Description of the Related Art
In general, inkjet printheads are devices for printing a predetermined color image by ejecting small droplets of printing ink at a desired position on a recording sheet. Ink ejection mechanisms of an inkjet printer are generally categorized into two different types: a thermally-driven type, in which a heat source is employed to form bubbles in ink thereby causing an ink droplet to be ejected, and a piezoelectrically-driven type, in which an ink droplet is ejected by a change in ink volume due to deformation of a piezoelectric element.
A typical structure of a thermally-driven inkjet printhead is shown in FIG. 1. Referring to FIG. 1, an inkjet printhead includes a substrate 10, a passage forming layer 20 stacked on the substrate 10, and a nozzle layer 30 which is formed on the passage forming layer 20. An ink supply hole 51 is formed in the substrate 10. The passage forming layer 20 has an ink chamber 53 storing ink, and a restrictor 52 connecting the ink supply hole 51 and the ink chamber 53. The nozzle layer 30 has a nozzle 54 through which the ink is ejected from the ink chamber 53. Also, a heater 41 for heating ink in the ink chamber 53 and an electrode 42 for supplying current to the heater 41 are provided on the substrate 10.
The ink ejection mechanism of the conventional thermally-driven inkjet printhead having the above-described configuration will now be described. Ink is supplied from an ink reservoir (not shown) to the ink chamber 53 through the ink supply hole 51 and the restrictor 52. The ink filling the ink chamber 53 is heated by a heater 41 consisting of resistive heating elements. The ink boils to form bubbles which expand so that the ink in the ink chamber 53 is ejected by a bubble pressure. Accordingly, the ink in the ink chamber 53 is ejected outside the ink chamber 53 through the nozzle 54 in the form of ink droplets.
The conventional thermally-driven inkjet printhead having the above-described configuration can be monolithically manufactured by photolithography, and the manufacturing process thereof is illustrated in FIGS. 2A through 2E.
Referring to FIG. 2A, a substrate 10 having a predetermined thickness is prepared, and a heater 41 for heating ink and an electrode 42 for supplying a current to the heater 41 are formed on the substrate 10.
As shown in FIG. 2B, a negative-type photoresist is applied to the entire surface of the substrate 10 to a predetermined thickness, and patterned in such a shape as to surround an ink chamber and a restrictor by photolithography, thereby forming a passage forming layer 20.
As shown in FIG. 2C, a space surrounded by the passage forming layer 20 is filled with positive-type photoresist, thereby forming a sacrificial layer S. In detail, the positive-type photoresist is applied to the entire surface of the substrate 10 to a predetermined thickness, and patterned, thereby forming a sacrificial layer S. Here, the positive-type photoresist is generally applied by spin coating, and the top surface of the applied positive-type photoresist is not planarized due to the centrifugal force. In other words, the positive-type photoresist bulges upward around the passage forming layer 20 due to the centrifugal force during spin coating, as indicated by the double-dashed line shown in FIG. 2C. If the uneven surface of the positive-type photoresist is patterned, the sacrificial layer S protrudes upward at its peripheral edges.
As shown in FIG. 2D, negative-type photoresist is applied to the passage forming layer 20 and the sacrificial layer S to a predetermined thickness, and patterned by photolithography, thereby forming a nozzle layer 30 having a nozzle 54.
Subsequently, as shown in FIG. 2E, the bottom surface of the substrate 10 is wet-etched to form an ink supply hole 51, and the sacrificial layer S is removed through the ink supply hole 51, thereby forming a restrictor 52 and an ink chamber 53 in the passage forming layer 20.
Referring back to FIG. 2D, when forming the nozzle layer 30 by applying negative-type photoresist to the sacrificial layer S, a projecting edge of the sacrificial layer S made of positive-type photoresist may react with a solvent contained in the negative-type photoresist, causing deformation or melting. Then, as shown in FIG. 2E, a cavity C is formed between the passage forming layer 20 and the nozzle layer 30.
FIG. 3 is a scanning electron microscope (SEM) photograph of a conventional inkjet printhead. Referring to FIG. 3, the passage forming layer 20 and the nozzle layer 30 are not perfectly adhered to each other due to existence of the cavity C formed between the passage forming layer 20 and the nozzle layer 30.
As described above, according to the conventional manufacturing method of an inkjet printhead, since the shape and dimension of the ink passage are not easily controlled, it is difficult to attain uniformity of the ink passage, and ink ejection performance of the printhead may deteriorate. Further, since the passage forming layer 20 and the nozzle layer 30 are not perfectly adhered to each other, the durability of the inkjet printhead is lowered.
Referring back to FIG. 2D, the negative-type photoresist applied to the sacrificial layer S is patterned by exposure, development and baking. During exposure, broadband UV light, including I-line (353 nm), H-line (405 nm) and G-line (436 nm), is usually used. Here, the H-line and G-line having a relatively long wavelength has a long penetration depth, affect both the negative-type photoresist forming the nozzle layer 30 and the positive-type photoresist forming the sacrificial layer S disposed under the nozzle layer 30. Also, when the positive photoresist which is most widely used is irradiated with UV light, a photosensitizer contained therein may be decomposed by light, producing nitrogen (N2) gas. The produced nitrogen gas expands during baking to lift the nozzle layer 30, resulting in deformation of the nozzle layer 30.
FIG. 4A is a plan view showing a state in which bubbles are generated in the sacrificial layer, and FIG. 4B is a photograph showing a cross section of a portion where the bubbles are generated. Referring to FIGS. 4A and 4B, nitrogen gas is generated in the sacrificial layer S made of the positive-type photoresist, and the nozzle layer 30 has deformed due to the nitrogen gas.